The present invention relates to a catalyst-coated membrane, a membrane-electrode assembly and a polymer electrolyte fuel cell comprising the same.
Since fuel cells (FC) have high power generation efficiency and are environmentally friendly, widespread use thereof as a distributed energy system is expected in the future. Particularly, polymer electrolyte fuel cells that use a polymer electrolyte having cations (hydrogen ions) are expected to be utilized in mobile units such as automobiles, distributed power generation systems and home cogeneration systems because they have high output density, they can operate at low temperatures and they can be made smaller.
Conventional polymer electrolyte fuel cells generate electricity and heat simultaneously by electrochemically reacting a fuel gas containing hydrogen and an oxidant gas containing oxygen such as air. FIG. 4 is a schematic sectional view illustrating a basic structure of a unit cell designed to be mounted in a conventional polymer electrolyte fuel cell. FIG. 5 is a schematic cross sectional view illustrating a basic structure of a membrane-electrode assembly (MEA) designed to be mounted in the unit cell 100 shown in FIG. 4. FIG. 6 is a schematic sectional view illustrating a catalyst-coated membrane (CCM) constituting the membrane-electrode assembly 101 shown in FIG. 5.
As shown in FIG. 6, in a catalyst-coated membrane 102, on each surface of a polymer electrolyte membrane 111 capable of selectively transporting hydrogen ions is formed a catalyst layer 112 composed of a hydrogen ion conductive polymer electrolyte and a catalyst-carrying carbon including a carbon powder and an electrode catalyst (e.g. platinum metal catalyst) carried on the carbon powder. As the polymer electrolyte membrane 111, polymer electrolyte membranes made of perfluorocarbonsulfonic acid such as Nafion (trade name) manufactured by E.I. Du Pont de Nemours & Co. Inc., USA are now widely used.
As shown in FIG. 5, a membrane-electrode assembly 101 is composed of catalyst layers 112 and gas diffusion layers 113 formed on the outer surfaces of the catalyst layers 112. The gas diffusion layer 113 is made of, for example, carbon paper treated for water repellency and having gas permeability and electron conductivity. The combination of the catalyst layer 112 and the gas diffusion layer 113 forms an electrode 114 (anode or cathode). As shown in FIG. 4, a unit cell 100 is composed of the membrane-electrode assembly 101, gaskets 115 and a pair of separator plates 116. The gaskets 115 are placed on the outer periphery of the electrodes and sandwich the polymer electrolyte membrane so as to prevent the supplied fuel gas and the supplied oxidant gas from leaking out and to prevent them from mixing with each other. The gaskets 115 are usually integrated in advance with the electrodes and the polymer electrolyte membrane. In some cases, the combination of the polymer electrolyte membrane 111, a pair of electrodes 114 (each electrode comprising the catalyst layer 112 and the gas diffusion layer 113) and gaskets 115 is referred to as “membrane-electrode assembly”.
On the outer surfaces of the membrane-electrode assembly 101 are placed a pair of separator plates 116 for mechanically fixing the membrane-electrode assembly 101. On the surface of the separator plate 116 in contact with the membrane-electrode assembly 101 are formed gas flow paths 117 for supplying a reaction gas (fuel gas or oxidant gas) to the electrode and removing a gas containing an electrode reaction product and unreacted reaction gas from the reaction site to the outside of the electrodes. Although the gas flow paths 117 may be formed independently of the separator plate 116, they are usually formed by providing grooves on the surface of the separator plate as shown in FIG. 4. On the other side of the separator plate 116 not in contact with the membrane-electrode assembly 101 is formed a cooling water flow path 118 by providing a groove by cutting.
As described above, a single unit cell constructed by fixing the membrane-electrode assembly 101 with the pair of separator plates 116 can produce an electromotive force of about 0.7 to 0.8 V at a practical current density of several tens to several hundreds mA/cm2 when the fuel gas is supplied to the gas flow path of one of the separator plates and the oxidant gas is supplied to that of the other of the separator plates. Polymer electrolyte fuel cells, however, are usually required to produce a voltage of several to several hundreds volts when used as power sources. For this reason, in practice, the required number of unit cells are connected in series to give a stack for use.
In order to supply the reaction gas to the gas flow paths 117, there is required a manifold in which pipes for supplying the reaction gas are branched into a corresponding number of separator plates and the branched pipes are directly connected to the gas flow paths on the separator plates. Particularly, a manifold in which external pipes for supplying the reaction gas are directly connected to the separator plates is called “external manifold”. There is another type of manifold called “internal manifold”, which has a simpler structure. An internal manifold is composed of apertures formed in the separator plates having gas flow paths formed thereon. The inlet and outlet apertures are connected with the gas flow path. The reaction gas is supplied to the gas flow path directly from the aperture.
The gas-diffusion layer 113 has the following three functions: (1) to diffuse the reaction gas so as to uniformly supply the reaction gas from the gas flow path of the separator plate 116 formed on the outer surface of the gas diffusion layer 113 to the electrode catalyst in the catalyst layer 112; (2) to rapidly carry away water produced by the reaction in the catalyst layer 112 to the gas flow path; and (3) to transfer the electrons required for the reaction or the produced electrons. As such, the gas diffusion layer 113 is required to have high reaction gas permeability, high water drainage capability and high electron conductivity.
Typically, in order to impart gas permeability to the gas diffusion layer 113, a conductive substrate having a porous structure made of carbon powder, pore-forming material, carbon paper or carbon cloth having a developed structure is usually used. In order to impart water permeability, a water repellent polymer as typified by fluorocarbon resin or the like is dispersed in the gas diffusion layer 113. In order to impart electron conductivity, the gas diffusion layer 113 is formed using an electron conductive material such as carbon fiber, metal fiber or carbon fine powder. On the surface of the gas diffusion layer 113 in contact with the catalyst layer 112 may be formed a water repellent carbon layer including a water repellent polymer and a carbon powder.
As for the catalyst layer 112, it has the following four functions: (1) to supply the reaction gas supplied from the gas diffusion layer 113 to the reaction site in the catalyst layer 112; (2) to transfer the hydrogen ions required for the reaction on the electrode catalyst or generated hydrogen ions; (3) to transfer the electrons required for the reaction or generated electrons; and (4) to accelerate the electrode reaction by its high catalytic performance and its large reaction area. As such, the catalyst layer 112 is required to have high reaction gas permeability, high hydrogen ion conductivity, high electron conductivity and high catalytic performance.
Typically, in order to impart gas permeability to the catalyst layer 112, a carbon fine powder or pore-forming material having a developed structure is used to form a catalyst layer having a porous structure and a gas channel. In order to impart hydrogen ion permeability (hydrogen ion conductivity), a polymer electrolyte is dispersed in the vicinity of the electrode catalyst in the catalyst layer 112 so as to form a hydrogen ion network. In order to impart electron conductivity, an electron conductive material such as carbon fine powder or carbon fiber is used as the carrier for the electrode catalyst to form an electron channel. In order to improve catalyst performance, a catalyst body including a carbon fine powder and a finely particulate electrode catalyst having a particle size of several nm carried on the carbon fine powder is densely dispersed in the catalyst layer 112.
For the commercialization of polymer electrolyte fuel cells, various attempts have been made to improve the performance of the membrane-electrode assembly 101 and the catalyst-coated membrane 102.
For example, in an attempt to suppress the degradation due to decomposition of a polymer electrolyte membrane, Japanese Laid-Open Patent Publications Nos. Hei 10-92444 (patent publication 1) and 2003-59512 (patent publication 2) propose techniques aimed at enhancing the mechanical strength and heat resistance of a polymer electrolyte membrane. Specifically, the above documents propose a method for physically reinforcing a polymer electrolyte membrane using a core material, and a method for chemically enhancing the durability of a polymer electrolyte membrane.
As for a catalyst layer, Japanese Laid-Open Patent Publications Nos. Hei 08-88008 (patent publication 3), 2003-303596 (patent publication 4) and 2004-47454 (patent publication 5) focus on the catalyst layer having a uniform monolayer structure from a polymer electrolyte membrane to a gas diffusion layer, and propose catalyst layers having novel structures from various points of view. Specifically, the above documents propose a method for changing the porosity of the catalyst layer in an attempt to prevent flooding due to product water and a method for changing the ratio of polymer electrolyte within a catalyst layer in an attempt to ensure proton conductivity in the near surface of the polymer electrolyte membrane.
Moreover, Japanese Laid-Open Patent Publication No. 2002-298860 (patent publication 6) proposes a technique to form a catalyst layer including a plurality of layers having different amounts of electrolyte in an attempt to obtain a catalyst layer having a good balance of proton conductivity and gas diffusibility. In an attempt to supply a reaction gas in a good condition regardless of the humidification conditions to improve the initial characteristics of the fuel cell, Japanese Laid-Open Patent Publication No. 2004-192950 (patent publication 7) proposes a technique to increase the porosity of a catalyst layer from the polymer electrolyte membrane side of the catalyst layer to the gas diffusion layer side of the same. Specifically, this patent publication investigates the amount of the polymer electrolyte relative to that of the catalyst in the catalyst layer.
The above-listed prior art publications, however, are essentially intended to improve the initial cell characteristics of the fuel cells and the mechanical strength and heat resistance of the polymer electrolyte membranes, and therefore no sufficient investigation has been made on a membrane-electrode assembly and a catalyst-coated membrane in the case of using the fuel cell for a long period of time, or on the improvement of durability and cycle life characteristics of the fuel cells.
To be more specific, even if the durability of a polymer electrolyte membrane is reinforced based on the techniques according to the above patent documents 1 and 2, the degradation of the membrane-electrolyte assembly as well as that of the catalyst-coated membrane cannot be sufficiently prevented when the fuel cell is operated for a long period of time. Accordingly, there still existed room for improvement in realizing a membrane-electrode assembly and a catalyst-coated membrane which have long service life and high efficiency.
The above patent documents 3 to 7 are silent on the technique to design a catalyst layer from the standpoint of enhancing the durability of a membrane-electrode assembly and a catalyst-coated membrane. Thus, there still existed room for improvement in realizing a membrane-electrode assembly and a catalyst-coated membrane which have long service life and high efficiency.
In short, the fuel cells according to the patent documents 1 to 7 still had room for improvement in terms of durability and cycle life characteristics.
In view of the above, an object of the present invention is to provide a catalyst-coated membrane and a membrane-electrode assembly which are capable of suppressing the degradation of a polymer electrolyte membrane for a long period of time even when the fuel cell is repeatedly operated and stopped and suitable for easily and surely achieving a polymer electrolyte fuel cell capable of sufficiently preventing the decrease in the initial characteristics and having excellent durability. Another object of the present invention is to provide a polymer electrolyte fuel cell, using the catalyst-coated membrane and the membrane-electrode assembly of the present invention, which can sufficiently prevent the decrease in the initial characteristics, exhibit sufficient cell performance for a long period of time and has excellent durability.